This invention relates to water tube boilers fitted with burners which are intended to combust gaseous or liquid fuels where low levels including ultra low levels of emissions of gaseous oxides of nitrogen (NO.sub.x) are required. Low levels of NO.sub.x is defined as concentrations less than 50 parts per million parts and ultra low NO.sub.x is defined as concentrations of less than 10 parts per million parts in combustion gases that are discharged to the atmosphere.
Oxides of Nitrogen (NO.sub.x) are unwanted atmospheric pollutants that are formed by high temperature chemical reactions between nitrogen in combustion air or in fuel and oxygen in combustion air. For boilers, NO.sub.x are combustion formed gases generated by utilizing gaseous, liquid or solid fuels for the generation of steam where temperatures in the highly turbulent flames approach the adiabatic temperature.
Several techniques of combustion modification are important to control the level of NO.sub.x in boiler off-gases and all rely on a means to reduce maximum flame temperature. All of the means of combustion modification rely on the integration of boiler design with burner characteristics to produce the lowest possible flame temperature. This integration involves setting a relationship between the amount of radiant heat absorbing surface of the boiler tube walls making up the combustion chamber and the combustion volume devoted to a single burner. Conventional water tube boilers, which have non-partitioned combustion chambers, have a geometric relationship between the volume contained by the water-cooled walls of the combustion chamber and the surface of the walls. As dimensions between walls are increased to produce larger combustion volumes required for greater capacities, the amount of surface of the walls increases, but not at the same proportion. Volume increases are greater than the attendant increase in radiant heating surface.
Flame temperatures (which are inversely influenced by the amount of radiant surface) are greater at higher capacity if, by design, the volume of the combustion chamber relative to heat input is held constant. Or, if the ratio of radiant surface to heat input is held constant, combustion volume increases unnecessarily and the overall dimensions of the boiler will increase.
The utilization of clean gaseous fuels, such as natural gas, and clean liquid fuels, such as distillate fuel oil, to generate steam is well known for convenience, less restrictive design and few operational problems. Also, using clean gaseous fuels is encouraged because the amount of photo-chemical and other air pollutants discharged into the atmosphere will be lower. One of the air pollutants of primary concern are chemical compounds of nitrogen and oxygen identified as NO.sub.x and usually reported chemically as NO.sub.2. NO.sub.x is photo-chemical contributing to smog and acid rain. Experience has shown that the average temperature at which combustion takes place and also the maximum temperature that occurs when a fuel is burned have direct relationships to the amount of NO.sub.x that will be formed. This means that higher flame temperatures within a boiler combustion chamber will increase the amount of NO.sub.x formed and discharged from the boiler.
This invention provides a combustion chamber design that utilizes well-known principles of radiant heat transfer combined with novel arrangements of the boiler's water tubes to absorb heat from fuels as they are being burned so as to lower combustion temperatures and thus, prevent formation of significant quantities of NO.sub.x.
The relationship between furnace walls heat absorbing surface and combustion volume is different with the novel design since there are a number of individually partitioned combustion chambers in parallel to each other. A separate burner is fitted to each combustion chamber. The design offers a way to have reasonably similar relationships of combustion volume and radiant heat absorbing surface for any boiler capacity. Additionally, the separate chambers will increase the radiant heat absorbing surface for the combustion volume normally seen for conventional boilers, thus, it offers a means for naturally decreasing maximum flame temperatures and thermally produced gaseous emissions.
Applicant is aware of the following U.S. patents: French 2,529,078; Behr 2,561,839; Druham 2,860,612; Vorkauf 2,988,063; Fujii 3,198,177; Beggs 3,221,711; Golibruzuch 3,245,395; Kendall, et al 4,154,568; Kendall, et al 4,204,829; Schreiber, et al 4,318,392; Schreiber, et al 4,412,523; Kendall, et al 4,492,185; Kendall, et al 4,494,485; Kesselring, et al 4,519,770; Krill, et al 4,543,940; Kendall 4,658,762; Kendall, et al 4,664,620; Kendall et al 4,730,559; and, Kendall, et al 4,809,672.